About 2% children develop a type of relative blindness in one eye which is known as functional amblyopia. The structures in the visual system that are affected and the ways in which they are altered are not fully known. Treatment of functional amblyopia is ineffective or only partially effective for most patients. The objective of the proposed research is to ascertain the basic mechanisms in the visual system that produce functional amblyopia, and to use this information to develop new methods of treating, and perhaps preventing, this form of blindness. Present evidence clearly indicates that a sensory (inhibitory) mechanism is operating in most cases of functional amblyopia. Recent evidence suggests that the oculomotor abnormalities exhibited by amblyopic eyes may not be a consequence of the reduced vision, but rather may contribute to the acuity reduction and may, in some cases, comprise the primary deficit. The proposed research is directed specifically at establishing the relative roles of sensory (inhibitory) and motor mechanisms in the genesis of functional amblyopia. Auditory biofeedback will be used to control the position of the amblyopic eye while visual acuity is measured at specified loci across the retina. Comparison with similar measurements for the normal eye will be used as a measure of the degree of sensory inhibition present in the amblyopic eye. Measurement of the oculomotor behaviour of amblyopic eyes will be made for stationary and moving targets to identify specific eye-movement error-signal deficits. Results from the proposed research will permit formulation of a classification system for functional amblyopia which should aid in selecting and designing specific treatment procedures.